Homologous genetic recombination will be studied using a defined in vitro system and purified proteins from bacteria. The proteins to be studied are the recA protein, and to a lesser extent the single-stranded DNA binding protein (SSB), both of E. coli. The recA protein promotes key steps in homologous genetic recombination in E. coli and is the prototype of a growing class of reconbinases detected in a variety of prokaryotic and eukaryotic cells. The in vitro system involves a DNA strand exchange reaction (a displacement of one strand of a duplex DNA with another) which mimicks the in vivo reaction but is much easier to study. The long range goal of this project is to elucidate the chemical mechanisms by which this and similar proteins promote genetic recombination. Since the recA protein-promoted DNA strand exchange reaction requires ATP hydrolysis and exhibits a unique polarity, these studies should also provide information about how chemical energy is kinetic, physical and genetic techniques available for biochemical analysis The health relatedness of this work derives from the importance of genetic recombination in many basic biological processes. Especially important in this regard is the central role of genetic recombination events in the generation of antibody diversity and in transformation of mammalian cells by mutagenic agents or by DNA tumor viruses.